MCAT Bio 2.1
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Replication occurs on both the leading strand and the lagging strand at the same time.
Each strand of the original DNA becomes the template strand for the new DNA strand.
Leading Strand is replicated continuously whereas the lagging strand is replicated in segments called Okazaki fragments.
Reads the template strand 3'->5' but synthesizes the NEW strand 5'->3' (new nucleotides are added on to the 3' end of the sugar preceding it).
Describe the process of DNA replication
Starts at the origin of replication...it opens up into a replication bubble...replication is occurring in 4 separate areas inside this bubble at the same time! Why?...DNA polymerase III requires an RNA primer to be already annealed to the DNA strand in order to begin synthesizing...helicase goes in front of DNA polymerase III and unwinds the DNA to allow room for replication...DNA polymerase I has 5' -> 3' exonuclease activity so it comes in at the end and plows off all the primers (plus maybe a few more nucleotides) and replaces them with DNA nucleotides...and then ligase comes in a binds the okazaki fragments together...throughout this whole process, the strain on DNA caused by the unwinding DNA is released by topoisomerases and gyrases.
Has its own RNA template that it uses to do reverse transcriptase and create DNA nucleotides to add on to the end of a DNA polymer to protect it from degradation.
RNA vs DNA
- RNA has a 2' hydroxyl group and DNA doesn't
- RNA is usually single-stranded and DNA is usually double-stranded
- Uracil (RNA) vs. Thymine (DNA)
- RNA exists in 3 forms vs. DNA which is in 1 form
- RNA exits the nucleus but DNA does not
- 1. RNA polymerase attaches to the promotor region of the DNA template strand.
- 2. DNA is unwound to form a replication bubble. 3. RNA polymerase starts synthesizing the pre-mRNA via RNA complementary base pairing.
- 3. The replication bubble continues down the DNA with the DNA behind it binding back together.
- 4. Once the RNA polymerase gets to the termination sequence, it releases from the template strand as well as the pre-mRNA.
- 5. Splicesome splices out introns in pre-MRA.
- 6. Poly-A-tail attached to 3' end.
- 7. 5' cap added. 8. mRNA is finally ready to be shipped out of the nucleus.
Names for the strand of DNA being TRANSCRIBED
- Anti-sense strand
- Anti-coding strand
- Template strand
Names for the strand of DNA NOT being TRANSCRIBED
- Coding Strand
- Sense Strand
- these are EXACTLY the same as the RNA strand but with thymine instead of Uracil.
- Watch out for the 5'3' convention of writing RNA!
3 ways to regulate Transcription
1) Rate of transcription-> RNA has a short half-life so genes must be constantly transcribed in order to continue protein production
2) Activators and Repressors-> certain substances upregulate transciption and others downregulate it...(Classify the following as activators or repressors for the Lac operon...lactose, glucose, Lac I protein, cAMP, CAP)
3) Permanent or Semi-permanent Repression-> methylation or other covalent modification that prevents transcription (When a promoter region of DNA is methylated it often silences the genes)
Special aspects about the genetic code
1) Degenerative= more than one codon for a single amino acid (the 3' nucleotide in the codon and the 5' nucleotide in the anticodon do NOT always need to match perfectly...wobble base pairs)
2) Unambiguous= there is never more than one amino acid coded for by a particular codon
1 start codon and 3 stop codons
- Start: AUG (methionine)
- Stop: UAA, UAG, UGA
Codons vs. Anticodons
Codons are the 3-nucleotide segments on the mRNA strand (they are read in the 5'->3' direction)...anti-codons are the 3-nucleotide segments that are on the complementary tRNA that brings in the appropriate nucleotide (they are also read in the 5'->3' direction but remember that they are complementary and pair up with codons in the anti-parallel direction).
Ribosome binds upstream of the initial codon and reads the mRNA strand in the 5'->3' direction(the region btwn the ribosome binding site and the intial codon is called the 5' UTR)...when it reaches the Start codon it begins bringing in tRNA's with the appropriate amino acids and catalyzes the polymerization of a polypeptide.
Location of translation
Germ cells vs. Somatic cells
Germ cells produce gametes (meiosis) whereas Somatic cells produce normal cells (mitosis) throughout the body.
Germ (sex) cells much worse because they can be transmitted to offspring while somatic (body) cells don't get transmitted.
6 types of DNA mutations
- 1) Point= a single nucleotide is changed
- 2) Missense mutation= a single nucleotide is changed that causes a new codon in the mRNA strand that codes for a different amino acid
- 3) Nonsense mutation= a single nucleotide is changed that causes a premature stop codon
- 4) Neutral Mutation= the mutation has NO effect on the individual's fitness (the amino acid might still be the same or it might be different but still have none to very little effect on protein function and structure and thus little effect on the organism's fitness)
- 5) Silent mutation= a mutation that has NO effect on the amino acids of the protein...it might be a mutation that creates a new codon that codes for the same protein or it might be a mutation in an intron
- 6) Frameshift mutation= an insertion or deletion causes a shift in the reading frame so that everything after it is new or distorted codons
Spread of diseased cells from one area of the body to another.
Proto-oncogenes are genes that help regulate cell growth.
A gene that has been mutated so that its products, in one way or another, cause cancer
Homozygous for a given trait (may be either dominant or recessive).
P1, F1, F2 generations
- P1 = you
- F1 = your kids
- F2 = your grandkids
Alternate forms of the same gene.
Mutagen vs Carcinogen
Both cause mutations but carcinogens always cause cancer while mutagens may be benign. All carcinogens are mutagens but not all mutagens are carcinogens.
Law of Segregation
Alleles segregate independently of one another when forming gametes.
Homologous pairs divide (1 to each gamete) during meiosis.
Law of Independent Assortment
Genes located on different chromosomes assort independently.
When homologous pairs line up on the metaphase plate they arrange themselves in a random fashion.
Important MCAT genetics convention
Whenever the genotype of an individual is not stated you should assume that it is HOMOZYGOUS DOMINANT!!!!...if the individual is a carrier or is affected (homozygous recessive usually) they will tell you.
The number of individuals in the population carrying the allele who actually express the phenotype.
The varying expression of disease symptoms despite identical genotypes.
A characteristic/trait for which the phenotype depends on many genes to help dictate it.
A single allele that affects many traits.
When cells within the same person have different genetic make-up.
When expression of a gene depends on the parent of origin. Sometimes both copies of a gene are not expressed and the "active" gene is dependent on who the gene came from.
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